The present invention relates to calenders for finishing paper stock, and in particular to an improved filled supercalender roll and method of making the same.
It is common to use one or more pairs of calender rolls to process paper. The rolls have smooth surfaces and a paper web is fed into a nip or nips therebetween, with the resultant compression of the paper and smoothing of its surface imparting a gloss to the surface. The rolls may be made of metal such as steel, but a pair of metal rolls often will not provide satisfactory operation when relatively thin paper is calendered. Because of slight variations in the concentricities of the rolls and/or as a result of slight variations in the thickness of the paper web, substantial differences in pressure are applied to various portions of the passing web. Some parts of the web therefore receive insufficient pressure from the rolls for adequate smoothing of the web surface, while other parts receive too much pressure which results in excessive compacting of the web and a decrease in its opacity.
To overcome the disadvantages of calendering using metal rolls only, a supercalender often is used. A supercalender has a plurality of vertically stacked metal rolls between which are composition or filled supercalender rolls that form nips with the metal rolls. Supercalender rolls commonly have a roll body formed by compressing together discs of fibrous material such as cotton, paper or the like, that are in side-by-side relationship on a shaft. The surface of the roll body is finish machined so that it is smooth and cylindrical.
The metal rolls of a supercalender may be heated to obtain desired results, and in the calendering operation such direct heat and/or heat from friction or other causes heats the supercalender rolls. The weight of the rolls, which is usually augmented by additional force, causes an indentation in the supercalender rolls at the nips formed with the metal rolls, resulting in a speed differential in the nips between the surfaces of the filled supercalender and metal rolls. The paper web to be supercalendered is fed through the nips, so that the sliding, frictional polishing action caused by the speed differential produces a high gloss on the surface of the paper.
The resilience of a supercalender roll produces a fairly equal pressure across the width of the paper web, thus eliminating the problems encountered in using metal rolls only. Such resilience is a matter of degree, and while the surface of the supercalender roll is hard, it is substantially more resilient than the surface of a metal roll. In order to first permit the removal of imperfections such as wrinkles or tears from the paper web, which can damage the surface of the supercalender roll, supercalenders are not normally used in line with paper machines. Even when used off line, however, it often happens that web imperfections damage the surface of the supercalender roll.
In addition to the surface of a conventional supercalender roll being susceptible to damage by imperfections in the paper web, another problem is its inability to withstand high nip temperatures, particularly those due to uneven loading in the nip and the shock impact of being repeatedly compressed by metal rolls. Cotton and paper fibers on the supercalender roll disintegrate in this environment, just beneath the roll surface, leading to a condition known as "burning." The burning of the roll body causes its surface to become uneven and cracked, so that the roll is rendered unfit for service. The supercalender roll must then be removed from the calender stack for remachining to remove such defects, whether caused by burning or marking due to imperfections in the paper web or foldovers due to web breaks. Consequently, there is an ever occurring need for refinishing, which is an expensive and time consuming operation.
Previous efforts to improve the life of filled supercalender rolls have included water cooling their shafts, making their filling from asbestos, either alone or in conjunction with cellulose, and using so-called heat resistant fibers, e.g., ramie in conjunction with cellulose. Such expedients have helped, but have not satisfactorily solved the problem.